Dynamic dampers are widely used to reduce vibrations that occur in machines. A dynamic damper is made up of movable mass member and a spring with an eigenvalue of vibration that corresponds to a target frequency of vibration to be damped. The vibration is reduced by an inertial force of the movable mass member by utilizing the phenomenon that the movable mass member moves in an opposite phase to a vibration at the eigenvalue.
In this case, an eigenvalue of vibration ω, an eigenfrequency f, a spring constant K, and a mass M of the movable mass member have the following relationship.
                    f        =                              ω                          2              ⁢                                                          ⁢              π                                =                                    1                              2                ⁢                                                                  ⁢                π                                      ⁢                                          K                M                                                                        [                  Equation          ⁢                                          ⁢          1                ]            
A generally used dynamic damper vibrates at an eigenfrequency that is determined by a ratio of the mass M of the movable mass member of the dynamic damper to the spring constant K in an opposite phase to inputted vibration, to reduce the vibration by utilizing an inertial force of the movable mass member. Thus, it is necessary to use one dynamic damper per one frequency of vibration to be damped. When using a plurality of dynamic dampers to handle a plurality of frequencies, the dynamic dampers may interfere with each other when their eigenfrequencies are close to each other, which may even possibly amplify the vibration instead of reducing the vibration.
When damping vibrations and noises that occur in a rotating machine entailing torque variations, a representative example of which is an engine of an automobile, the target frequency of vibration to be damped varies in accordance with the rotational speed of the engine. As a result, there are several frequencies of vibration that should be dealt with.
In view of this, there has been proposed a device in which a member made of magnetic viscoelastic elastomer (MRE) is used as a spring to realize a dynamic damper with variable eigenfrequency (see patent document 1). This is a variable dynamic damper technique that varies stiffness of rubber by mixing magnetic material into the rubber and controlling the intensity of a magnetic field applied to the rubber.